Innate immunity in animals and plants can be activated by highly conserved pathogen/microbe- associated molecular patterns (PAMPs/MAMPs), such as bacterial flagellin and lipopolysaccharides (LPSs). Recent studies provide evidence that plant stomata, pores in the epidermis of plants, function in innate immunity against the entry of bacteria into the plant. Specifically, stomata close in response to Escherichia coli O157:H7 and Pseudomonas syringae pv. tomato (Pst) DC3000 or bacterial PAMPs. Perception of PAMPs by guard cells, the pair of cells that forms the stomatal pore, requires a flagellin receptor as well as production and signaling of the plant hormones abscisic acid and salicylic acid. The virulent plant pathogen Pst DC3000 produces the phytotoxin coronatine (COR) to suppress stomatal closure as a virulence strategy. Collectively, these results suggest an important role of stomata in modulating the interactions of plants with plant and human pathogenic bacteria in the phyllosphere (the leaf surface). The long-term goal of this project is to elucidate the signal transduction pathway leading to stomate- based defense and to dissect the mechanism by which the bacterial toxin COR inhibits this novel host defense. The central hypotheses to be tested are that (i) stomate-based defense is an integral part of the plant's natural immune response to restrict the entry of bacteria, including human pathogenic bacteria, and (ii) to be successful, plant pathogens must rely on specific virulence factors or environmental conditions to suppress stomate-based defense. An integrative approach involving molecular genetics, microscopy, and biochemical and pathogenesis assays will be used. The proposed research will contribute to the fundamental knowledge of innate immunity and bacterial pathogenesis in plants and will increase our understanding of the molecular basis of plant-human pathogen interactions in the phyllosphere. Human pathogen contamination of fresh vegetables is a major public health concern and a potential bio-terrorist threat as a source of food poisoning. PUBLIC HEALTH RELEVANCE: This proposal describes basic research on the molecular basis of innate immunity of the model plant Arabidopsis to plant and human pathogenic bacteria. Food contamination with human pathogens is a major concern to public health. This research has the potential to provide a mechanistic understanding of plant defense and human pathogen contamination of fresh produce and to guide future development of appropriate prevention measures.